This section is intended to provide a background or context to the disclosed embodiments that are recited in the claims. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and claims in this application and is not admitted to be prior art by inclusion in this section.
Watermarks are substantially imperceptible signals embedded into a host content. The host content may be any one of audio, still image, video or any other content that may be stored on a physical medium or transmitted or broadcast from one point to another. Watermarks are designed to carry auxiliary information without substantially affecting fidelity of the host content, or without interfering with normal usage of the host content. For this reason, watermarks are sometimes used to carry out covert communications, where the emphasis is on hiding the very presence of the hidden signals. Other widespread applications of watermarks include prevention of unauthorized usage (e.g., duplication, playing and dissemination) of copyrighted multi-media content, automatic content recognition (ACR), proof of ownership, authentication, tampering detection, content integrity verification, broadcast monitoring, transaction tracking, audience measurement, triggering of secondary activities such as interacting with software programs or hardware components, communicating auxiliary information about the content such as caption text, full title and artist name, or instructions on how to purchase the content, and the like. The above list of applications is not intended to be exhaustive, as many other present and future systems can benefit from co-channel transmission of main and auxiliary information.
Watermark system design involves balancing a number of different requirements, such as imperceptibility, robustness, security, false positive rate, processing load, payload size, etc. In some applications of watermarks, such as copy management applications, the watermarks that are embedded in a host content must often be extracted in consumer products (e.g., media players, copiers, etc.) that access and use the content. As a result of limited computational resources (e.g., memory, real estate on silicon, processing cycles, etc.) available to watermark extractors within such devices, these applications often create challenges in meeting all of the above noted requirements of the watermarking system since an improvement in one requirement often comes at the expense of another requirement. For example, the robustness of watermark extraction (i.e., a measure of how well an extractor can extract embedded watermarks in the presence of noise and other impairments in the host content and/or watermarking channel) can often be improved by increasing the number of extraction attempts at, for example, small offsets from a particular location of interest of the host content. An increase in the number of extraction attempts, however, increases both the processing load on the extractor and the probability of false watermark extractions. As another example, an increase in payload size typically comes at the expense of reduced robustness and/or increased false positive rate.
In should be noted that in the present application, the term “watermark extraction” is used to describe the operations that result in the extraction of a watermark payload, i.e. the bits or symbols of the message carried by the watermark, which is different from removal or erasure of watermarks from the host content. Alternatively, watermark extraction can be called watermark detection, but sometimes watermark detection is used to indicate the detection of the presence of a watermark within a host content without actually extracting the payload carried by that watermark.